Week 7-Long Piston With Cam

TRANSIENT STRUCTURAL ANALYSIS ON A PISTON AND CAM MECHANISM USING ANSYS WORKBENCH

OBJECTIVE

1. To perform transient structural analysis on a piston and cam mechanism for the following case study,

1. Case-1: Frictionless contact.
2. Case-2: Frictional contact, coefficient of friction=0.1.
3. Case-3: Frictional contact, coefficient of friction=0.2.

2. To find out Equivalent Stress, Directional Deformation and Equivalent Elastic Strain for all the three cases and compare the results.

1. THEORY

1.1 Cam Mechanism:

The transformation of one of the simple motions, such as rotation, into any other motions is often conveniently accomplished by means of a cam mechanism. A cam mechanism usually consists of two moving elements, the cam and the follower, mounted on a fixed frame. Cam devices are versatile, and almost any arbitrarily-specified motion can be obtained. In some instances, they offer the simplest and most compact way to transform motions.

A cam may be defined as a machine element having a curved outline or a curved groove, which, by its oscillation or rotation motion, gives a predetermined specified motion to another element called the follower. The cam has a very important function in the operation of many classes of machines, especially those of the automatic type, such as printing presses, shoe machinery, textile machinery, gear-cutting machines, and screw machines. In any class of machinery in which automatic control and accurate timing are paramount, the cam is an indispensable part of mechanism. The possible applications of cams are unlimited, and their shapes occur in great variety. Cams classified according to their basic shapes:

1. A plate cam, also called a disk cam or a radial cam.
2. A wedge cam.
3. A cylindric cam or barrel cam.
4. An end cam or face cam.

1.2 Cylindrical cam or barrel cam: 

A cylindrical cam or barrel cam is a cam in which the follower rides on the surface of a cylinder. In the most common type, the follower rides in a groove cut into the surface of a cylinder. These cams are principally used to convert rotational motion to linear motion parallel to the rotational axis of the cylinder. A cylinder may have several grooves cut into the surface and drive several followers. Cylindrical cams can provide motions that involve more than a single rotation of the cylinder and generally provide positive positioning, removing the need for a spring or other provision to keep the follower in contact with the control surface. Applications include machine tool drives, such as reciprocating saws, and shift control barrels in sequential transmissions, such as on most modern motorcycles.

In this project a long piston and cam mechanism similar to barrel cam is simulated for transient structural analysis for three different cases of contact. In case (1), the contact type is frictionless whereas, for case (2) and case (3), the contact type is frictional with coefficient of friction being 0.1 and 0.2 respectively.

2. ANALYSIS SETUP

2.1 Geometry:

Fig.2.1.1 3D model of piston and cam.

The given 3D model of piston and cam assembly is imported into SpaceClaim. It consists of a barrel, follower and cam.

Fig.2.1.2 Stiffness behavior.

The stiffness behavior of barrel and cam is assigned as ‘rigid’ and follower is assigned as ‘flexible’.

2.2 Material Properties:

Fig.2.2 Material property details of piston and cam.

The material assigned for the long piston and cam assembly is structural steel.

2.3 Connection Details:

2.3.1 Contact details:

a. Contact between cam follower and barrel.

b. Contact between cam follower and barrel cam.

Fig.2.3.1 Contact details of piston and cam mechanism.

Contact between, (a) cam follower (contact body) and barrel (target body), (b) cam follower (contact body) and barrel cam (target body) are assigned as frictionless contact for case (1). The type of contact assigned for case (2) and case (3) is frictional contact with coefficient of friction being 0.1 and 0.2 respectively.

Note: The analysis setup of only case (1) is demonstrated.

2.3.2 Joint Details:

a. Fixed support applied to barrel.                                                                    b. Revolute joint specified for barrel cam.

c. Translational joint specified for follower with reference to barrel.

Fig.2.3.2 Joint details of piston and cam mechanism.

The outer cylindrical surface of the barrel is fixed. In order to rotate the barrel cam to guide the follower for predefined motion, the revolute type of joint is specified along Z-axis to the one end of the barrel cam with connection type being body-ground. To slide the follower cam in linear motion with reference to barrel along X-axis, translational joint is specified to cam follower with connection type being body-body.

2.4 Meshing:

a. Body sizing of follower cam.                                                                          b. Face sizing of contact region of barrel cam.

c. Meshed model.

Fig.2.4 Meshing details of piston and cam mechanism.

The mesh size of the cam follower is refined to 3 mm by using body sizing option. The mesh size of the contact region of barrel cam is set as 3 mm using face sizing option. The total number of nodes and elements generated are 9918 and 5347 respectively.

Note: The academic version of software has the problem size limit of 128k nodes or elements.

2.5 Boundary Conditions:

2.5.1 Analysis settings:

a. Analysis setting for step 1.                                           b. Analysis setting for step 2 to step 8.

Fig.2.5.1 Analysis settings.

In the analysis settings the number of steps considered is 9 and auto time stepping is set to ‘On’ which is defined by ‘time’. For step 1, the initial, minimum and maximum time step is considered as 0.1s, 5e-2s and 0.2s respectively. For step 2 to step 9, the minimum and maximum time step is 5e-2s and 0.3s respectively. In the solver controls, solver type is chosen as ‘Program controlled’, weak springs is set to ‘Program controlled’ and large deflection is set to ‘On’. In the non-linear controls the stabilization is set to ‘constant’ with energy dissipation ratio being ‘0.1’ and activation for first substep is set to ‘Yes’. Under the output controls, all the required results are set to ‘Yes’.

2.5.2 Boundary condition:

Fig.2.5.2 Joint load applied to barrel cam.

The joint load is applied to barrel cam to rotate it in clockwise direction from 0⁰ to 270⁰, with an increment of 30⁰ in each step.

3. RESULTS AND DISCUSSIONS

3.1 Case-1: Frictionless contact.

a. Directional deformation (Y-axis).                                                                              b. Directional deformation (X-axis).

c. Equivalent (v-m) Stress.                                                                                         d. Equivalent Elastic Strain.

3.2 Case-2: Frictional contact, coefficient of friction=0.1.

a. Directional deformation (Y-axis).                                                                            b. Directional deformation (X-axis).

c. Equivalent (v-m) Stress.                                                                                        d. Equivalent Elastic Strain.

3.3 Case-3: Frictional contact, coefficient of friction=0.2.

a. Directional deformation (Y-axis).                                                                            b. Directional deformation (X-axis).

c. Equivalent (v-m) Stress.                                                                                        d. Equivalent Elastic Strain.

3.4 Comparison of Results:

From the table, it is observed that as the coefficient of friction is increased, the value of Max. deformation, v-m stress and equivalent elastic strain is also increased, because as coefficient of friction is increased the frictional resistance to motion is also increased. Hence, for case (3) with frictional contact having coefficient of friction as 0.2 has the highest value for max. deformation, v-m stress and equivalent elastic strain compared to case (2) and case (1).

4. ANIMATION OF RESULTS:

4.1 Case-1: Frictionless contact.

a. Directional deformation (Y-axis).                                                                      b. Directional deformation (X-axis)

c. Equivalent (v-m) Stress.                                                                                  d. Equivalent Elastic Strain.

4.2 Case-2: Frictional contact, coefficient of friction=0.1.

a. Directional deformation (Y-axis).                                                                      b. Directional deformation (X-axis)

c. Equivalent (v-m) Stress.                                                                                  d. Equivalent Elastic Strain.

4.3 Case-3: Frictional contact, coefficient of friction=0.2.

a. Directional deformation (Y-axis).                                                                      b. Directional deformation (X-axis)

c. Equivalent (v-m) Stress.                                                                                  d. Equivalent Elastic Strain.

CONCLUSION

The transient structural analysis on a piston and cam mechanism was carried out successfully for the following cases,

1. Case (1): Frictionless contact.
2. Case (2): Frictional contact, coefficient of friction=0.1.
3. Case (3): Frictional contact, coefficient of friction=0.2.

The maximum directional deformation (X-axis), Equivalent (v-m) stress and Equivalent elastic strain developed in case (3) is highest compared to case (2) and case (1), because as coefficient of friction is increased the frictional resistance to motion is also increased.